Backward wave prevention for a twt helix

ABSTRACT

A method of preventing deleterious backward wave oscillations in a high powered traveling wave tube. A loss, introduced at selected frequencies, effects the backward oscillation prevention. This method inherently provides a high heat transfer capability since the loss may be introduced by the moving fluid that dissipates the heat generated in the fluid. The selection of a particular fluid and the dimensions of the hollow support ceramic fluid flow tubes result in selective loss in the range of 6-8 GHz in a TWT designed for operation between 2 and 4 GHz.

United States Patent n91 Winslow i Dec. 25, 1973 1 1 BACKWARD WAVE PREVENTION FOR A TWT HELIX [75] lnventor: Lester M. Winslow, Alexandria, Va.

[73] Assignee: The United States of America as represented by the Secretary of the Navy, Washington, DC.

22 Filed: Jan. 24, 1973 [21] Appl. No.: 326,218

[52] US. Cl. 315/35, 313/30, 333/22 F [51] Int. Cl. H01j 25/34 [58] Field of Search 315/35; 313/30,

[56] References Cited UNITED STATES PATENTS 5/1972 Smith et a1 3l5/3.5

11/1971 Marchese 315/3.5 7/1967 Landsbergen 315/35 3,231,780 1/1966 Feinstein 315/35 X 3,536,952 10/1970 Findley 3,320,471 5/1967 Minis 313/30 X Primary ExaminerRudolph V. Rolinec Assistant Examiner-Saxfield Chatmon, Jr. Attorney-R. S. Sciascia et a1.

[ ABSTRACT A method of preventing deleterious backward wave oscillations in a high powered traveling wave tube. A loss, introduced at selected frequencies, effects the backward oscillation prevention. This method inherently provides a high heat transfer capability since the loss may be introduced by the moving fluid that dissipates the heat generated in the fluid. The selection of a particular fluid and the dimensions of the hollow support ceramic fluid flow tubes result in selective loss in the rangeof 6-8 61-12 in a TWT designed for operation between 2 and 4 (11-12.

3 Claims, 4 Drawing Figures PATENTEDDECZS 1975 I sum 1 or 2 PATENTED DEEZ 5 I875 PIERCE NORMALIZED LOSS PARAMEgER d 0 iv SHEET 2 [IF 2 0.5 MOD AQU US SODI CHL DE FREQUENCY IN GHZ fosci FIG. 4.

. I 1 BACKWARD WAVE PREVENTION FOR AZTWT HELIX BACKGROUND OF THE INVENTION High powered helix type traveling wave tubes are becoming more widely used in commercial and military amplification purposes. A problem encountered in the art has been that backward wave oscillations are-propagated back along the electron beam. This deleterious situation causes undesired output power to be generated at unwanted frequencies and a reduction in the output power at the desired frequency. Prior art methods to reduce backward .wave oscillations in general have tended to greatly reduce the available r.f. power.

Many attempts have been-made to preventbackward wave oscillations in :helix tubes, and they include the use of phase velocity variations, perturbations, or the introduction of stop bands'at the BWO (Backward Wave Oscillation) frequency. Eowever, it is generally admitted that these methods are substantially'inefl'ective to suppress the oscillations in designs which allow peak powers exceeding 3 kilowatts to be generated efficiently. Although some methods of helix derived circuits are able to generate high peak power, they are generally limited in bandwidth. Also, many oscillation prevention techniques which utilize lossy elements use a loss in the vacuum surrounding thebeam. As the lossy element-absorbs power-the temperature rises and gases may tend to evolve from the lossy element. This deleterious situation may cause adverse effectssuch as cathode poisoning or electron beam defocusing.

Thus considering such drawbacks and limitations, 1 have developed amethod of preventing-backward oscillations in a TWT by the use of various fluids employed in conjunction with the ceramic support tubes of the helix TWT. Not only does this invention provide a stop band to selectively :limit backward oscillations but also effectively provides for heat removal from the TWT system. Furthermore .the adverse effects of the evolved gases are overcome by providing for power absorption outside the vacuum.

SUMMARY A well known wave interaction structure is modified to advantageously increase the r.f. lossat certain frequencies. The r.f. loss created at these frequencies provides a -method or preventing backward wave'oscillations. The frequency and the amount of the loss desired may be controlled bythe selection of various fluids and .the particular dimensions of the ceramic support fluid flow tubes. Furthermore, a very high heat transfer capability is obtained in conjunction with the oscillation prevention. Thus the elimination of the need to conduct heat from a lossy element through a .vacuum barrier to a convection cooled heat sink.

OBJECTS It is an object of the invention to prevent backward wave oscillations in a traveling wave tube.

Another object of this invention is to introduce r.f.

Another object of the invention is to provide various fluids and ceramic support fluid flow tube dimensions to control the amount of loss.

Other objects and advantages of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings wherein:

THE DRAWINGS quency for various fluids.

DETAILED DESCRIPTION Referring to the drawings and more particularly FIG 1,,a section of a traveling wave tube 10 is shown having a vacuum envelope 12 formed of a nonmagnetic material. Coaxially disposed within envelope 12 is a sleeve 14 formed of two semicylindrical portions best seen in FIG. 2, joined at diametrically opposed longitudinal seams which are both brazed 'to form a high-pressure vacuum seal 18. Metal sleeve 14 is supported in its axial position within envelope 12 by a pair of longitudinal dividers or blocks 20 shaped on the outside to match the inside contour of vacuum envelope l2 and shaped on the inside thereof to matchthe outside surface of sleeve 14. On the outside surface of blocks 20, a groove 24 is milled which receives brazing material to form a highpressure fluid seal with the wall of envelope 12 when heated in the furnace. Blocks 20, vacuum envelope 12,

and metal sleeve 14 define a pair of semiannular cavities 26 and 28 which are fluid tight, with respect to one another and function as intake and exhaust fluid manifolds.

A helix 30 of metal tubing, such as annealed copper, is axially disposed within metal sleeve 14. A series of holes aligned in a pair of diametrically opposed longitudinal rows along the outside of the-helix is formedcommunicating with the interior of the tubing. Thus each turn of the helix has a pair of apertures formed in the outside wall thereof and the apertures are disposed in a pair of diametrically opposed rows running the length of the helix.

Looking now at FIG. 3, another traveling wave tube is shown and includes a pair of ceramic pipes 46 and 48 disposed longitudinally-within vacuum envelope 12 on opposite sides of helix 30. The interiors of pipes 46 and 48 communicate with the interior of helix 30 via a conduit 49 formed by a series of apertures formed therethrough at each half turn of the helix and a matching series of apertures formed through pipes 46 and 48. A braze 50 surrounds the junction of pipes 46 and 48 with the helix to mechanically join the helix to the two ceramic pipes. This establishes a series of fluid-tight junctions such that fluid employed in'this invention may be pumped under pressure into ceramic pipe 46, flows into the helix at each half turn, and exits directly into ceramic pipe 48. The assembly is inserted within vacuum envelope l2 and the device functions similarly to that shown in FIGS. 1 and 2, although the dielectric loading is somewhat higher in the embodiment shown in FIG. 3.

In operation, a radio frequency wave is introduced into helix 30 at an inlet microwave coupling (not shown). A beam of electrons from an electron gun (not shown) is directed axially through the middle of the helix and is collected at the exit and by a collector anode (also not shown). The radio frequency wave traveling along helix 30 is amplified by interaction with the electron beam traveling down the axis of the helix 30 in a manner well-known in the art. The amplifier wave is then collected at a conventional coupler (not shown). The hollow ceramic tubes 32 and 34 pass fluid from envelope 12 to the helix 30. This geometry causes an added lumped capacitive susceptance every half turn on the helix. A lumped conductance, the value of which is dependent upon the selection of a particular fluid, is electrically in parallel with a lumped capacitance. The lumped capacitance is due to ceramic tube 34 and its presence appears to cause an absorption band to occur in the structure when the phase shift between lumped susceptances is equal to 11/2 radians. The presence of the lumped conductance at or near the absorption band-edge causes the r.f. loss to increase rapidly. This increase prevents backward wave oscillations. Backward Wave Oscillations are prevented when the normalized loss d exceeds a value of about 1.5.

The fluid may be admitted to intake manifold 26 through a fluid line show schematically at 42 running from a heat exchanger (not shown) and a pump (also not shown). The coolant fills and pressurizes intake manifold 26 and is admitted to one side of each turn of helix 30 by means of ceramic tubes 32 which serve as a fluid conduit between manifold 26 and helix 30. The fluid then flows around each half turn of helix 30 and is exhausted to exhaust manifold 28 by means of ceramic tubes 34. The now warm fluid is then withdrawn from exhaust manifold 28 by outlet fluid line shown schematically at 44 and cycled back to the previously mentioned heat exchanger. Although the route of the fluid flow has been described it should be apparent that the invention may be practiced with a non-moving fluid at the expense of losing the heat removal capability as disclosed herein.

Referring to FIG. 4, the graphs of Pierce normalized loss parameter d vs. frequency for various fluids are shown. The Figure shows a very selective loss occurring at frequencies above 7 GB. The backward wave oscillation frequency for the structure shown is about 8.0 GHZ. It will be noted that although the curve for water is higher than desired in the operating band it is quite selective above 6 GHz and therefore is effective for backward wave oscillation prevention. That is to say the most advantageous condition shown in FIG. 4 is that of the acetone curve which was plotted for a struc ture having a ceramic support inner diameter of 0.34 inch. A modification to the support ceramic structure 34 reduces the loss in the pass band and greatly increases the selectivity so that the curve obtained for acetone can be attained by changing the ceramic dimensions for water. Specifically, changing the inner diameter of the ceramic supports from 0.034 inches to 0.020 inches the curve for water approaches the acetone curve shown in FIG. 4. Therefore, a careful manipulation of the inner diameter size of the ceramic supports and the choice of various fluids provides many combinations of particular selectivity.

As mentioned above the thermal capability of the circuit is very high. For example, at 3.0 Hz the circuit can dissipate greater than 5 kilowatts of r.f. power using water as the lossy fluid element. The power is absorbed into the moving fluid.

Obviously many modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

I claim:

1. In a traveling-wave tube of the type wherein a beam of electrons flows along the axis of a helically shaped, hollow conductor through which fluid flows and an electromagnetic wave propagates, whereby interaction occurs between the beam and the wave, the improvement which comprises:

means for preventing backward wave oscillations in the tube by providing at the upper 3 DB point a Pierce normalized parameter d in the order of L5 in the range of the backward oscillation frequency.

2. The combination of claim 1 wherein said backward wave oscillations preventing means comprises a plurality of tubular ceramic supports connected to the helically shaped conductor so as to transmit the fluid to and from said conductor, wherein said tubular supports have a predetermined inner dimension which is related to the particular fluid flowing within said supports.

3. The combination of claim 2 wherein the fluid is selected from the group consisting of water, methyl alcohol and acetone. 

1. In a traveling-wave tube of the type wherein a beam of electrons flows along the axis of a helically shaped, hollow conductor through which fluid flows and an electromagnetic wave propagates, whereby interaction occurs between the beam and the wave, the improvement which comprises: means for preventing backward wave oscillatioNs in the tube by providing at the upper 3 DB point a Pierce normalized parameter d in the order of 1.5 in the range of the backward oscillation frequency.
 2. The combination of claim 1 wherein said backward wave oscillations preventing means comprises a plurality of tubular ceramic supports connected to the helically shaped conductor so as to transmit the fluid to and from said conductor, wherein said tubular supports have a predetermined inner dimension which is related to the particular fluid flowing within said supports.
 3. The combination of claim 2 wherein the fluid is selected from the group consisting of water, methyl alcohol and acetone. 